The present invention relates to a brake booster and more particularly to a brake booster for an automotive vehicle operated by the differential of pressure between a vacuum and atmospheric pressure.
From German patent DE-AS 2,345,314 a brake booster for an automotive vehicle is known which is operated by the differential of pressure between a vacuum and atmospheric pressure. This brake booster comprises a vacuum casing, a movable wall disposed therein and a control valve, the movable wall including a diaphragm dividing the vacuum casing into a vacuum chamber of constant vacuum and a working chamber of different pressures and acting on a push rod guided in the axial direction of the brake booster. On the side of the vacuum casing close to the control valve, the casing end wall includes several bolts serving to secure the brake booster to the splash wall of an automotive vehicle. On the other side of the vacuum casing where the end of the push rod extends out of the casing end wall, the master cylinder is mounted. As a rule, this mounting is likewise by means of bolts to bolt the master-cylinder flange to this casing end wall.
In an arrangement of this type, the vacuum casing is conventionally of sheet steel. As a result of the pressure differential between the vacuum and the atmospheric pressure, pneumatic forces act on the vacuum casing. These forces, which somewhat compress the vacuum casing, have to be taken up by the components of the vacuum casing. To limit the amount of deformation of the vacuum casing, it is necessary to provide the casing components with an appropriate strength. These designs are, however, not economical because of the resultant increase in the material costs in addition to being of a high weight. Thus, such an arrangement is unable to comply with the vehicle manufacturers' demands for the lowest possible weight of brake systems without the safety and operability of the brake booster being impaired.
The arrangement disclosed in the present invention is of particular importance for a mechanically controlled brake booster utilizing the pressure differential between a vacuum and atmospheric pressure and comprising a vacuum casing having at least one movable wall disposed therein, the movable wall dividing the vacuum casing into a vacuum chamber and a working chamber and acting upon a push rod guided in the axial direction of the brake booster, with means for fastening the brake booster to a splash wall being provided on the vacuum casing on a casing end wall where a rod for the introduction of force extends into the vacuum casing, and further means for fastening a master cylinder being provided on the opposite casing end wall. The vacuum casing of such a brake booster as it is described, for example, in the above cited German Patent DE-AS 2,345,314, is subjected to high forces, and this is the main problem as pointed out below.
As the brake pedal is depressed, the force transmitted by the brake pedal and the booster force are active on the push rod. The total force resulting therefrom acts on the piston of the master cylinder, the resistance of the compressed hydraulic fluid in the master cylinder necessitating, however, the presence of an opposed force component maintaining the master cylinder approximately in its position. This reaction force is transferred from the master cylinder via the sheet-metal components of the vacuum casing to the splash wall where the pedal support is usually fastened.
On actuation of the brake booster, the reaction force results in an elongation of the vacuum casing in an axial direction and, consequently, in a displacement of the master cylinder which adds to the lost travel of the brake pedal. Because of the brake-lever transmission ratio, the elongation of the vacuum casing is increased by a multiple and transmitted to the brake pedal. The elongation is still further increased by the pneumatic forces acting on the vacuum casing. Their effects have to be taken into consideration to a still greater extent if the casing components are of reduced wall thickness.